Adsorber aftertreatment system having dual adsorbers

ABSTRACT

The present invention provides for an NOx adsorber aftertreatment system for internal combustion engines which utilizes adsorber catalysts arranged in parallel. The exhaust flow from the engine is divided in a predetermined ratio between the two catalysts during lean operation (e.g. 50-50). At a predetermined regeneration time (for example, when the adsorber catalyst is 20% full), the exhaust gas flow is reduced through the parallel leg that is to be regenerated (e.g., 20% through the leg to be regenerated, 80% of the flow to the other leg). A quantity of hydrocarbon is injected into the reduced-flow leg in order to make the mixture rich. Since the flow has been reduced in this leg, only a small fraction of the amount of hydrocarbon that would have been required to make the mixture rich during full flow is required. This will result in a substantial reduction in the fuel penalty incurred for regeneration of the adsorber catalyst. Once the leg has been regenerated, the flow distribution between the parallel legs is reversed, and the other catalyst leg is regenerated while the other side (which is now clean) receives the majority of the exhaust flow. Another advantage of the present invention is that since NOx is being stored in one leg while the other leg is being regenerated, the regeneration operation can be performed for a longer period of time, resulting in greater regeneration efficiency. Once both catalyst legs have been regenerated, the exhaust flow is adjusted back to normal (e.g. 50-50) until the catalysts are again ready for regeneration and reduction.

TECHNICAL FIELD OF THE INVENTION

The present invention generally relates to internal combustion enginesand, more particularly, to an NOx adsorber aftertreatment system forinternal combustion engines.

BACKGROUND OF THE INVENTION

As environmental concerns have led to increasingly strict regulation ofengine emissions by governmental agencies, reduction of nitrogen-oxygencompounds (NOx) in exhaust emissions from internal combustion engineshas become increasingly important. Current indications are that thistrend will continue.

Future emission levels of diesel engines will have to be reduced inorder to meet Environmental Protection Agency (EPA) regulated levels. Inthe past, the emission levels of US diesel engines have been regulatedaccording to the EPA using the Federal Test Procedure (FTP) cycle, witha subset of more restrictive emission standards for California via theCalifornia Air Resources Board (CARB). For example, the Tier II emissionstandards, which are being considered for 2004, are 50% lower than theTier I standards. Car and light truck emissions are measured over theFTP 75 test and expressed in gm/mi. Proposed Ultra-Low Emissions Vehicle(ULEV) emission levels for light-duty vehicles up to model year 2004 are0.2 gm/mi NOx and 0.08 gm/mi particulate matter (PM). Beginning with the2004 model year, all light-duty Low Emission Vehicles (LEVs) and ULEVsin California would have to meet a 0.05 gm/mi NOx standard to be phasedin over a three year period. In addition to the NOx standard, a fulluseful life PM standard of 0.01 gm/mi would also have to be met.

Traditional methods of in-cylinder emission reduction techniques such asexhaust gas recirculation (EGR) and injection rate shaping by themselveswill not be able to achieve these low emission levels required by thestandard. Aftertreatment technologies will have to be used, and willhave to be further developed in order to meet the future low emissionrequirements of the diesel engine.

Some promising aftertreatment technologies to meet future NOx emissionstandards include lean NOx catalysts, Selective Catalytic Reduction(SCR) catalysts, and Plasma Assisted Catalytic Reduction (PACR). Currentlean NOx catalyst technologies will result in the reduction of engineout NOx emissions in the range of 10 to 30 percent for typicalconditions. Although a promising technology, SCR catalyst systemsrequire an additional reducing agent (aqueous urea) that must be storedin a separate tank, which opens issues of effective temperature range ofstorage (to eliminate freezing) as well as distribution systems thatmust be constructed for practical use of this technology. PACR issimilar to lean NOx in terms of reduction efficiency but is moreexpensive due to plasma generator. These technologies, therefore, havelimitations which may prevent their use in achieving the new emissionsrequirements.

NOx adsorber catalysts have the potential for great NOx emissionreduction (60-90%). The NOx adsorber is one of the most promising NOxreduction technologies. During lean-burn operation of the engine, thetrap adsorbs nitrogen oxide in the form of stable nitrates. Understoiciometric or rich conditions, the nitrate is thermodynamicallyunstable and the stored nitrogen oxides are released and subsequentlycatalytically reduced. Therefore, the operation cycle alternates betweenlean and rich conditions around the catalyst. During lean operation thecatalyst stores the NOx and during rich operation the NOx is releasedand reduced to N₂. However, to make the conditions around the catalystrich, a significant amount of hydrocarbon (HC) needs to be injected. Theamount of HC required for reduction is only a small fraction of thetotal hydrocarbon injected, resulting in a significant fuel penalty. Ifthe HC required to make conditions rich can be reduced, the fuel penaltycan be brought down substantially.

There is therefore a need for an engine aftertreatment system employingan NOx adsorber which reduces the fuel penalty associated with these.The present invention is directed toward meeting this need.

SUMMARY OF THE INVENTION

The present invention provides for an NOx adsorber aftertreatment systemfor internal combustion engines which utilizes adsorber catalystsarranged in parallel. The exhaust flow from the engine is divided in apredetermined ratio between the two catalysts during lean operation(e.g. 50-50). At a predetermined regeneration time (for example, whenthe adsorber catalyst is 20% full), the exhaust gas flow is reducedthrough the parallel leg that is to be regenerated (e.g., 20% throughthe leg to be regenerated, 80% of the flow to the other leg). A quantityof hydrocarbon is injected into the reduced-flow leg in order to makethe mixture rich. Since the flow has been reduced in this leg, only asmall fraction of the amount of hydrocarbon that would have beenrequired to make the mixture rich during full flow is required. Thiswill result in a substantial reduction in the fuel penalty incurred forregeneration of the adsorber catalyst. Once the leg has beenregenerated, the flow distribution between the parallel legs isreversed, and the other catalyst leg is regenerated while the other side(which is now clean) receives the majority of the exhaust flow. Anotheradvantage of the present invention is that since NOx is being stored inone leg while the other leg is being regenerated, the regenerationoperation can be performed for a longer period of time, resulting ingreater regeneration efficiency. Once both catalyst legs have beenregenerated, the exhaust flow is adjusted back to normal (e.g. 50-50)until the catalysts are again ready for regeneration and reduction.

In one form of the invention, an internal combustion engineaftertreatment system for treating exhaust gases exiting an engine isdisclosed, the system comprising a sulfur trap having a sulfur trapinput operatively coupled to the engine exhaust and having a sulfur trapoutput, a catalytic soot filter having a soot filter input operativelycoupled to the sulfur trap output and having a soot filter output, avalve system having a valve input operatively coupled to the soot filteroutput, a first valve output and having a second valve output, a firstadsorber having a first adsorber input operatively coupled to the firstvalve output and having a first adsorber output, a second adsorberhaving a second adsorber input operatively coupled to the second valveoutput and having a second adsorber output, and a diesel oxidationcatalyst having a DOC input operatively coupled to the first and secondadsorber outputs and having a DOC output.

In another form of the invention, an internal combustion engineaftertreatment system for treating exhaust gases exiting an engine isdisclosed, the system comprising a catalytic soot filter having a sootfilter input operatively coupled to the engine exhaust and having a sootfilter output, a sulfur trap having a sulfur trap input operativelycoupled to the soot filter output and having a sulfur trap output, avalve system having a valve input operatively coupled to the sulfur trapoutput, a first valve output and having a second valve output, a firstadsorber having a first adsorber input operatively coupled to the firstvalve output and having a first adsorber output, and a second adsorberhaving a second adsorber input operatively coupled to the second valveoutput and having a second adsorber output.

In another form of the invention, an internal combustion engineaftertreatment system for treating exhaust gases exiting an engine isdisclosed, the system comprising a first adsorber having a firstadsorber input operatively coupled to engine exhaust and having a firstadsorber output, a second adsorber having a second adsorber inputoperatively coupled to the engine exhaust and having a second adsorberoutput, and an igniter having an igniter output operatively coupled tothe first and second adsorber inputs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a first preferred embodimentsystem of the present invention.

FIG. 2 is a schematic block diagram of a second preferred embodimentsystem of the present invention.

FIG. 3 is a process flow diagram illustrating a preferred embodimentprocess of the present invention.

DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiment illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, and alterations and modifications in theillustrated device, and further applications of the principles of theinvention as illustrated therein are herein contemplated as wouldnormally occur to one skilled in the art to which the invention relates.

Referring to FIG. 1, there is illustrated a schematic block diagram of afirst preferred embodiment of the present invention. The system isdesigned to remove NOx compounds from the exhaust stream of an internalcombustion engine 12, such as a diesel engine. The exhaust produced bythe engine 12 exits the exhaust manifold 14 of the engine and is passedthrough an optional sulfur trap 16. NOx adsorber catalysts are extremelysensitive to the level of sulfur in the fuel. The fuel and thelubrication oil of the engine contain sulfur and therefore sulfur-oxygencompounds (SOx) are contained in the exhaust gas. This SOx is adsorbedinto the NOx adsorber and reduces its capacity. Unlike NOx, SOx does notregenerate under rich conditions within the operating temperature rangeof the engine. Eventually the adsorber is filled up with sulfate andbecomes inactive. The optional sulfur trap 16 may therefore be used totrap SOx compounds before they reach the NOx adsorbers downstream.

The output of the sulfur trap 16 may be passed through an optionalcatalytic soot filter 18 in order to trap any diesel soot particulatematter that may be entrained in the exhaust gases. In addition totrapping diesel soot particulate matter by physical filtering, thecatalytic soot filter also acts as an oxidation catalyst by the additionof precious metal catalysts which reduce the volatile organic fractionof the soot material by the catalyzed oxidation reaction (e.g.C+Oxidant→CO). A sensor 20 may be placed at the output of the sootfilter 18 in order to measure the temperature and air/fuel (A/F) ratio(lambda) of the exhaust stream. The output of the optional sensor 20 isprovided to an electronic engine control module 22.

The engine controller 22 is additionally coupled to the engine 12 forreading various engine sensor data, such as engine position sensor data,speed sensor data, air mass flow sensor data, fuel rate data, etc., asis known in the art. The engine controller 22 may further provide datato the engine 12 in order to control the operating state of the engine12, as is well known in the art.

The flow of exhaust leaving the soot filter 18 is controlled by aproportional control 3-way valve 24. As is known in the art, aproportional control 3-way valve may be used to divide the flow of a gasstream into two separate paths, wherein the percentage of the total gasflow being directed to either path is controllable. In the embodiment ofFIG. 1, the proportional control 3-way valve 24 is coupled to the enginecontroller 22 in order to control the relative proportions of exhaustgas flow routed to either output of the valve 24.

The two outputs of the valve 24 are coupled to the respective inputs ofa pair of NOx adsorbers (catalytic converters) 26 and 28. Therefore, byproviding control signals from the engine controller 22 to theproportional control 3-way valve 24, the percentage of the total exhaustflow from the engine 14 entering either the adsorber 26 or the adsorber28 may be precisely controlled. A fuel injector 30 is positioned toinject a measured quantity of fuel (hydrocarbon) into the exhaust gasflow entering the adsorber 26. Similarly, a second fuel injector 32 ispositioned to inject a quantity of fuel into the exhaust gas flowentering adsorber 28. Both injectors 30, 32 are controlled by the enginecontroller 22 and are supplied with fuel from a pump 34 supplied by thevehicle fuel tank 36. Preferably, the fuel pump 34 is a low-costdiaphragm-type fuel pump. Two igniters 38 are provided to ignite thefuel being injected by the injectors 30, 32 under the control of theengine controller 22.

Because the exhaust flow is reduced in the adsorber leg beingregenerated, the amount of reductant required to burn off the oxygenreduces. The concentration of reductant required for reduction remainsthe same, but this amount is a small fraction of the total reductantduring full exhaust flow. It will be appreciated that any flow ratiosmay be utilized during reduction and regeneration and during normalflow, even though exemplary flows are used herein for illustrativepurposes. The optimum flow ratios for any given system will depend uponthe particular system configuration.

The exhaust gases exiting the adsorbers 26 and 28 are combined togetherbefore being input to an optional diesel oxidation catalyst 40. Due tothe pulse injection of relatively large quantities of reductant(normally hydrocarbon) for short periods during regeneration of the NOxadsorbers 26, 28 of the present invention, some unburned hydrocarbon canslip through the adsorber catalyst. The use of a diesel oxidationcatalyst 40 downstream of the adsorbers 26, 28 virtually eliminateshydrocarbon emission from the tailpipe. Such catalysts contain preciousmetals in them that reduce the activation energy of hydrocarboncombustion, such that the unburned hydrocarbon is oxidized to carbondioxide and water. The exhaust gases exiting the diesel oxidationcatalyst 40 may then exit the vehicle. An optional NOx sensor 42 may beplaced between the adsorbers 26, 28 and the diesel oxidation catalyst 40in order to directly measure the NOx levels leaving the adsorbers 26 and28. The output of the optional NOx sensor 42 is provided to the enginecontroller 22.

Referring now to FIG. 2, there is illustrated a second preferredembodiment of the present invention. The second embodiment of thepresent invention is similar to the first embodiment illustrated in FIG.1, and like reference designators refer to like components. In thesecond embodiment, the proportional control 3-way valve is replaced witha pair of 2-way valves 50 and 52. Valve 50 controls the flow of exhaustgases into the adsorber 26, while valve 52 controls the flow of exhaustgases into adsorber 28. Each of the valves 50, 52 is coupled to theengine controller 22 for control thereby. The injectors 30, 32 may belocated downstream (as shown) or upstream of the valves 24, 50, 52.

The valves 50, 52 may comprise either variable flow rate control valvesor may comprise valves having a fixed number of flow rate settings. Forexample, if the aftertreatment system design dictates that the relativeflow between adsorbers 26, 28 will always be 20-80 during regeneration,then the valves 50, 52 may have discrete settings that will allow theengine controller 22 to switch them between reduced flow (20%) and maxflow (80%) settings in order to achieve the desired flow reduction inone of the adsorbers 26, 28. Optionally, the valves 50, 52 may havevariably adjustable flow rates, such that the engine controller 22 caninfinitely adjust the flow percentage through each valve 50, 52 in orderto divide the exhaust flow between the adsorbers 26, 28 in any desiredproportion.

Referring now to FIG. 3, there is illustrated a preferred embodimentprocess of the present invention. The process begins at step 100, whichrepresents the steady state operation of the engine with exhaust gasflow split evenly between the adsorbers 26 and 28. At step 102, theengine controller 22 determines whether either of the adsorber 26, 28catalysts need be regenerated. The decision made at step 102 can be madeunder open-loop control, by using stored catalyst adsorption maps in theengine controller 22. These catalyst adsorption maps may bepredetermined using empirical data from laboratory tests utilizing thesame or similar engine and exhaust system. The regeneration decision atstep 102 may also be made under closed-loop control, wherein the enginecontroller 22 examines the data being produced by the NOx sensor 42which is proportional to the level of NOx being emitted at the output ofthe adsorbers 26, 28.

If step 102 determines that the adsorbers 26, 28 need to be regenerated(e.g. the adsorption efficiency has dropped to 80%), then the processcontinues at step 104 in which the flow of exhaust through the system iscontrolled such that the adsorber to be regenerated receives a reducedlevel of exhaust flow. For example, if the engine controller 22determines that adsorber 26 needs to be regenerated, then the flow ofexhaust through the adsorber 26 can be reduced to 20% of the totalexhaust flow, with the remaining 80% being routed through the adsorber28. The relative proportions of exhaust flow routed to either adsorberwill depend upon various system design parameters. The 20-80 splitdiscussed herein is for illustrative purposes only.

Control of the relative flow of exhaust gases through adsorbers 26 and28 is performed under control of the engine controller 22 (for example,based upon the engine sensor parameters being sent to the controller 22(engine position sensor, speed sensor, air mass flow sensor, fuel rate,etc.)) through operation of either the proportional control 3-way valve24 of the system of FIG. 1 or through control of the dual 2-way valves50, 52 of the system of FIG. 2, which are adjusted to achieve thecorrect predetermined exhaust flow velocity needed for regeneration ofthe aftertreatment system.

Once the correct flow velocity has been achieved through each of theadsorbers 26, 28, the process moves to step 106 in which the enginecontroller 22 determines the temperature and air/fuel ratio of theregeneration exhaust stream using the sensor 20. If the temperature ofthe exhaust stream is sufficient for regeneration of the catalysts(according to a predetermined temperature limit), then the processcontinues to step 110. If step 106 determines that the temperature ofthe regeneration exhaust stream needs to be raised, then the processcontinues at step 108 in which the engine controller 22 causes theigniter 38 to be activated in order to ensure ignition of theregeneration fuel injection.

At step 110, the fuel injector 30, 32 in the leg being regenerated isused to inject the required amount of fuel into the exhaust stream as areductant to completely regenerate the catalysts within the adsorber.The injectors 30, 32 are controlled by the engine controller 22. Theexhaust fuel injector 30, 32 is used to achieve a rich air/fuel ratio(lambda less than 1.0) in the regeneration stream. Because of thereduced amount of exhaust gas flowing through the regeneration leg, thequantity of fuel needed to be injected by the injector 30, 32 is greatlyreduced, thereby significantly reducing the fuel penalty associated withadsorber regeneration. This injected fuel will be ignited by thetemperature of the exhaust gas stream (possibly supplemented by theigniter 38) in order to facilitate regeneration of the adsorber.

Once regeneration of the leg is determined to be complete at step 112(e.g. after a predetermined amount of time has elapsed), the processcontinues at step 114, where the engine controller 22 determines if bothlegs of the system have been regenerated. If they have not, then theprocess continues at step 116, where the engine controller 22 operateseither the proportional control 3-way valve 24 or the 2-way valves 50,52 in order to route the majority of the exhaust gas flow to therecently regenerated leg and to reduce the amount of exhaust gasesflowing through the leg which is to be regenerated. The process is thenreturned to step 106 in order to regenerate the next leg. If, on theother hand, step 114 determines that both legs have been regenerated,then the process is returned to step 100 where the engine controller 22operates the proportional control 3-way valve 24 or the 2-way valves 50,52 in order to evenly split the exhaust gas flow through the adsorbers26, 28.

As detailed hereinabove, the adsorber regeneration cycle switches backand forth between the two sides of the exhaust as necessary in order tokeep the outlet exhaust stream purified of excessive emissions. It willbe appreciated that since dual exhaust streams are being utilized, theregeneration cycle of the adsorber does not necessarily have to beshort. During the entire time that the adsorber is being regenerated,the second adsorber is available for cleaning the majority of theexhaust gas stream. It should also be noted that the temperature of theregeneration exhaust gas stream may also be controlled by adjustment ofthe proportional control 3-way valve in conjunction with the igniter 38.By allowing slightly more exhaust gas to pass into the regeneration sideof the exhaust, the temperature thereof may be raised.

Besides the aforementioned advantages in adsorber regeneration, thearrangement of catalysts illustrated in FIGS. 1 and 2 of the presentinvention provides other benefits. Placing the catalytic soot filter 18before the adsorbers 26, 28 helps in multiple ways. The catalytic sootfilter 18 converts the NO in the exhaust stream to NO₂ which helps NOxstorage in the adsorber 26, 28. The catalytic soot filter 18 alsoprevents particulate matter from clogging the adsorber system and italso helps increase the temperature of the exhaust stream in order tomake the adsorber 26, 28 more efficient.

In another embodiment, the sulfur trap 16 may be placed downstream fromthe catalytic soot filter 18. By placing the catalytic soot filter 18upstream of the sulfur trap 16, the catalytic soot filter 18 willconvert SO₂ to SO₃, which is more readily trapped by the sulfur trap 16.

Therefore, the system illustrated and described herein is effective inaddressing all legislatively-controlled emissions including NOx, SOx andhydrocarbons. The adsorbers are used for reduction of NOx levels and aremore easily regenerated than in prior art systems. The sulfur trapremoves sulfur from the exhaust, making the operation of the adsorbermore efficient and longer lasting. The catalytic soot filter trapsparticulate soot from the exhaust stream. Finally, the diesel oxidationcatalyst cleans up any leftover hydrocarbons exiting the adsorbers,thereby allowing the exhaust emitted by the system of the presentinvention to meet or exceed the requirements of the various legislativebodies.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the-same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and that allchanges and modifications that come within the spirit of the inventionare desired to be protected.

We claim:
 1. An internal combustion engine aftertreatment system fortreating exhaust gases exiting an engine, the system comprising: asulfur trap having a sulfur trap input operatively coupled to the engineexhaust and having a sulfur trap output; a catalytic soot filter havinga soot filter input operatively coupled to the sulfur trap output andhaving a soot filter output; a valve system having a valve inputoperatively coupled to the soot filter output, a first valve output andhaving a second valve output; a first adsorber having a first adsorberinput operatively coupled to the first valve output and having a firstadsorber output; a second adsorber having a second adsorber inputoperatively coupled to the second valve output and having a secondadsorber output; and a diesel oxidation catalyst having a DOC inputoperatively coupled to the first and second adsorber outputs and havinga DOC output.
 2. The system of claim 1, further comprising: a supply offuel; a pump having a pump inlet operatively coupled to the supply offuel and having a pump outlet; a first fuel injector having a firstinjector input operatively coupled to the pump outlet and having a firstinjector output operatively coupled to the first adsorber input; and asecond fuel injector having a second injector input operatively coupledto the pump outlet and having a second injector output operativelycoupled to the second adsorber input.
 3. The system of claim 1, furthercomprising: An igniter operatively coupled to the first and secondadsorber inputs.
 4. The system of claim 1, further comprising: atemperature and lamda sensor having a sensor input operatively coupledto the soot filter output.
 5. The system of claim 1, further comprising:an NOx sensor having an NOx sensor input operatively coupled to thefirst and second adsorber outputs.
 6. The system of claim 1, wherein thevalve system comprises a proportional control 3-way valve.
 7. The systemof claim 1, wherein the valve system comprises a pair of 2-way valves.8. An internal combustion engine aftertreatment system for treatingexhaust gases exiting an engine, the system comprising: a catalytic sootfilter having a soot filter input operatively coupled to the engineexhaust and having a soot filter output; a sulfur trap having a sulfurtrap input operatively coupled to the soot filter output and having asulfur trap output; a valve system having a valve input operativelycoupled to the sulfur trap output, a first valve output and having asecond valve output; a first adsorber having a first adsorber inputoperatively coupled to the first valve output and having a firstadsorber output; and a second adsorber having a second adsorber inputoperatively coupled to the second valve output and having a secondadsorber output.
 9. The system of claim 8, further comprising: a dieseloxidation catalyst having a DOC input operatively coupled to the firstand second adsorber outputs and having a DOC output.
 10. The system ofclaim 8, further comprising: a supply of fuel; a pump having a pumpinlet operatively coupled to the supply of fuel and having a pumpoutlet; a first fuel injector having a first injector input operativelycoupled to the pump outlet and having a first injector outputoperatively coupled to the first adsorber input; and a second fuelinjector having a second injector input operatively coupled to the pumpoutlet and having a second injector output operatively coupled to thesecond adsorber input.
 11. The system of claim 8, further comprising: anigniter operatively coupled to the first and second adsorber inputs. 12.The system of claim 8, further comprising: a temperature and lamdasensor having a sensor input operatively coupled to the soot filteroutput.
 13. The system of claim 8, further comprising: an NOx sensorhaving an NOx sensor input operatively coupled to the first and secondadsorber outputs.
 14. The system of claim 8, wherein the valve systemcomprises a proportional control 3-way valve.
 15. The system of claim 8,wherein the valve system comprises a pair of 2-way valves.